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Blown start capacitor.

I am a refrigeration mechanic and am frustrated that I can't figure this problem out. It doesn't help that it's on my mother in laws heat pump.

Her system is a 3 year old R410a Heritage 15 American Standard electric variable speed air handler with 15 seer American Standard heat pump.

1) service call of lights flickering in house. When I arrive I notice that the start capacitor is very low (around 30 mfd). I replace and test the potential relay. Start windings drop out as they should. System runs great.

2) two weeks later they call me to say the lights are flickering again. I return to find the new start capacitor has literally blown up. Ohm out compressor windings and find 3.1, 2.2, 2.1. Suspect a Run to Start winding short. Replace compressor, potential relay, start capacitor, filter drier, and refrigerant. Start up and perform full system checklist. Everything is awesome.

3) today (about two weeks later) I go to their house to pick up my son and swing into the back yard to check things out with the heat pump. Rated compressor amp is 11.1. Actual is 8.9. I kill power to test the start capacitor and it measures 35-40 mfds. I restore power to the unit and it pulls LRA for about 2 seconds (56.7amps) before eventually starting up again.

Did you give the pressures enough time to equilizer before restoring power? Could be that the system is not equalizing before asct times out. Maybe the equilizer line on txv or bleed function of txv is restricted, or you got a Chinese start cap.

A start cap needs to have the resistor. It can be intermittently sticking pot contacts due to arcing.
The resistor allows the cap to bleed off after dropping out of the circuit. If it doesn't bleed off and the pot relay resets, while the cap is fully charged, it can weld contacts permanently or intermittently.
When the start cap remains in operation too long it will blow off the pressure relief, and likely dry up, losing capacitance. Remember, most electrolyte mixtures are water based. So, if it becomes overheated, hydrogen gas will be released and blow out the top.
The potential relay also has a directional arrow that must face upward for proper operation.

The relay is installed correctly however the cap doesn't have a bleed resister. Of course this begs the question, why sell a start cap without a bleed resistor? Thanks for the info. I will purchase a start cap with a bleed resistor. Any other suggestions would be great as well.

Sometimes they come in the box with spade clips, to be installed and removed when checking cap with a meter. If you have the resistor connected while checking the mfd rating, it will be off due to the resistors interference of the capacitors reactance.

Should have replaced the contactor when you did the compressor changeout. When you get that new start cap, pick up a contactor as well.

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The best way to add age to a capacitor is through over use. So.... A run cap will lose value when the current draw through it is really high. As for Start Caps.... Most after market 'boost' or kickstart caps have a resistor under the cap on top. As long as there is a time delay to ensure the pressures are actually equalized before it starts again (causing high current draw on startup and premature capacitor aging) and the capacitor actually is discharged before the system starts (with a resistor) and the system is not short cycling for some reason (over use again) you should be all set.

Some start caps do test out at a value other than the nameplate because the resistor throws off what the meter tells you (FYI) so dont test it on initial installation and say its busted because it reads off of the nameplate value.